The cellular transcriptional regulation is a complex biological process. One of the basic principles is the post-translation modification of histone proteins H2A/B, H3 and H4 that form the octameric histone core complex. The complex N-terminal modifications at lysine residues by acetylation or methylation and at serine residues by phosphorylation constitute part of the so-called “histone code” (Stahl & Ellis, Nature 403, 41-45, 2000).
In a simple model, acetylation of positively charged lysine residues decreases affinity to negatively charged DNA, thus transcription factors may be easily entered.
Histone acetylation and deacetylation is catalysed by histone acetyltransferases (HATs) and histone deacetylases (HDACs), respectively. The HDAC is associated with transcriptional repressor complexes, switching chromatin to a transcriptionally inactive, silent structure (Marks et al. Nature Cancer Rev 1, 189-202, 2001). The opposite holds true for certain HATs which are associated with transcriptional activator complexes. Three different classes of HDACs, located in the nucleus, have been described so far, namely, class I (HDAC 1-3, 8; Mr=42-55 kDa) sensitive towards inhibition Trichostatin A (TSA), class II (HDAC 4-7, 9, 10; Mr=120-130 kDa) sensitive to TSA, and class III (Sir2) which are quite distinct by their NAD+ dependency and TSA insensitivity.
Inhibitors of histone deacetylases (HDACs) constitute a new class of anticancer drugs with differentiation and apoptosis activity. By targeting histone deacetylases (HDACs), the HDAC inhibitors affect histone (protein) acetylation and chromatin structure, inducing a complex transcriptional reprogramming, exemplified by reactivation of tumor suppressor genes and repression of oncogenes. In addition to generating acetylation of N-terminal lysine residue in core histone protein, there are non-histone targets important for cancer cell biology, including heat-shock-protein (HSP90), tubulin or p53 tumor suppressor protein. Therefore, the HDAC inhibitors can be used for the treatment of cancers as well as inherited metabolic diseases, autoimmune diseases, etc., since the efficacy in animal models for inflammatory diseases, rheumatoid arthritis, and neurodegeneration has been shown.
Examples of histone deacetylase-mediated diseases include cell proliferative diseases including malignant tumors such as cancers, etc., autosomal dominant diseases such as Huntington's diseases, etc., inherited metabolic diseases such as cystic fibrosis, hepatic fibrosis, kidney fibrosis, pulmonary fibrosis, skin fibrosis, etc, autoimmune diseases such as rheumatoid arthritis, etc., acute and chronic neurological diseases such as diabetes, stroke, etc., hypertrophy such as cardiac hypertrophy, etc., hemorrhagic heart failure, amyotrophic lateral sclerosis, glaucoma, ocular diseases (associated with angiogenesis), Alzheimer's disease, etc.
HDAC inhibitors known up to now can be classified into four categories according to their structures: 1) short chain fatty acids (butyric acid, valproic acid); 2) hydroxamic acids (trichostatin A, SAHA, LBH-589); 3) cyclic peptides (desipeptide); and 4)benzamide (MS-275, MGCD-0103) (International Journal of Oncology 33, 637-646, 2008). These many HDAC inhibitors (SAHA, LBH-589, MS-275, etc.) effectively induce growth inhibition, differentiation, and apoptosis of various transformed cells in culture medium as well as in animal models (Marks, P. A et. al., Curr Opin Oncol. 2001. 13. 477-483), and some HDAC inhibitors such as SAHA, LBH-589, MS-275, etc. are clinically evaluated for the treatment of various cancers (Johnstone, R. W Nat. Rev. Drug Discov. 2002 1. 287-299). Typical examples of HDAC inhibitor compounds that are currently known include hydroxamate compounds, such as SAHA (U.S. Pat. No. 771,760, Zolinza, Vorinostat), PXD101 (WO 02/30879, Belinostat), LBH-589 (WO 02/22577, Panobinostat), and benzamide compounds such as MS-275 (EP8799) and MGCD0103 (WO 04/69823). Among these compounds, SAHA was approved on October 2006 and has been used for the treatment of CTCL (cutaneous T-cell lymphoma). Diseases for which medicine is efficacious have been additionally expanded, but it is known that there are drawbacks in terms of effectiveness and side effects (Cancer Res 2006, 66, 5781-5789).
That is, although many HDAC inhibitors have been reported to date, most of the HDAC inhibitors have drawbacks in terms of efficacy and side effects. Therefore, in order to overcome these drawbacks, there is a continuous need to develop an effective HDAC inhibitor with high selectivity and less side effects (Mol Cancer Res, 5, 981, 2007).